FRANKFORT - Kentucky plans to vaccinate at least 5,000 medical workers against smallpox, and perhaps as many as 10,000, if ordered by President Bush, a public health official said Tuesday.

The first to be vaccinated would make up the advance teams that check out possible cases of smallpox.

"The people who go investigate that ought to be vaccinated," said Dr. Rice Leach, the state's public health commissioner. "People likely to be taking care of the patient ought to be vaccinated against it. And if it turns out the patient did have smallpox, the vaccinators ought to be vaccinated."

Smallpox, caused by a virus, has not been seen in the United States since 1949.

Though the state has planned for it, mass vaccination of the public in advance of an actual outbreak is unlikely, Dr. Leach said.

"In the absence of a threat of widespread exposure ... , I'd be surprised if we vaccinated masses of people," he said. "If we had widespread disease that looked like it was going to spread all over the country, we have a contingency plan to vaccinate everybody."

Medical workers who volunteer would be vaccinated at about a dozen clinics around the state, Dr. Leach said.

A special needle is used to scratch a patch of skin, and the vaccine is applied. Side effects usually are benign - aches, fever and some swelling at the point of vaccination. But the vaccine is dangerous to some people, even fatal in rare cases, Dr. Leach said.